Issue 53 migrate from deprecated uiwebview

[Ingibjorg]

I removed the usage of UIWebView since it's a deprecated API and Apple has warned that they will not be accepting apps to the AppStore that use it (See https://github.com/seekshiva/react-native-remote-svg/issues/53)

When I enabled the use of WKWebView instead of UIWebView, I also had to remove the scalesPageToFit setting since it's not supported in WKWebView.

There's an issue with transparent backgrounds on WKWebView so the WebView displayed a white background before loading the SVG. I followed the workaround suggested here: https://github.com/react-native-community/react-native-webview/issues/318#issuecomment-470366316 to prevent the white background to show.